Lead-acid batteries are standard on most types of transportation vehicles including microhybrid vehicles. For example, lead-acid batteries are used to start the internal combustion engines of automobiles, trucks, and other equipment and to supply electricity for vehicle accessories. These battery requirements are known in the industry as SLI (Starting, Lighting and Ignition). Lead-acid batteries are also used in industrial stationary applications including emergency lighting and power supply systems with battery backup such as data networks, high-speed data transmission networks, wireless communication and cable TV systems.
There are two basic types of lead-acid battery designs: conventional flooded lead-acid and sealed Valve Regulated Lead-Acid (VRLA). VRLA batteries are sometimes referred to as an absorbed glass mat (AGM) battery. The performance requirements for these two types of lead-acid batteries vary significantly. It is well known that lead-acid batteries enjoy the best price/performance ratio for all energy storage devices available today.
Stationary applications are generally float applications, i.e., the cells are generally on float (i.e., an external voltage supply connected to the cells is held slightly above the cell potential to maintain charge), with an occasional need for a deep discharge when the main power source fails or is otherwise interrupted.
Other applications require repetitive deep discharges, down to a 80% depth of discharge or even somewhat greater. Suitable cells must thus be capable of enduring repetitive charge-deep discharge-charge cycling regimes for up to 500 cycles or even more. Indeed, it would be desirable to provide cells capable of enduring from 1,000 to 2,000 cycles or so.
Developing grid alloys that adequately satisfy the diverse requirements is difficult because stringent criteria must be satisfied regardless of the type of application. Suitable alloys must be capable of being cast into satisfactory grids and must impart adequate mechanical properties to the grid. Also, the alloys must impart satisfactory electrical performance to the cell in the intended application. Satisfactory alloys thus must impart the desired corrosion resistance, not result in thermal runaway (i.e., must not raise the tendency for the cell to lose water via gassing) and avoid premature capacity loss (sometimes referred to as “PCL”).
More particularly, suitable alloys must be capable of being cast into grids by the desired technique, i.e., the cast grids must be low in defects as is known (e.g., relative freedom from voids, tears, microcracks and the like). Such casting techniques range from conventional gravity casting (“book molds” or the like) to continuous processes using expanded metal techniques. Alternatively, grids may be punched.
The resulting grids need to be strong enough to endure processing into plates and assembly into cells in conventionally used equipment. Even further, suitable grids must maintain satisfactory mechanical properties throughout the expected service life. Any substantial loss in the desired mechanical properties during service life can adversely impact upon the cell performance as will be more fully discussed hereinafter.
Considering now the electrochemical performance required, the grid alloy for positive plates must yield a cell having adequate corrosion resistance. Yet, the use of a continuous direct casting process, for example, which may be desirable from the standpoint of economics, ostensibly can compromise corrosion resistance. Such continuous processes thus orient the grains in the grids, thereby making the intergranular path shorter and more susceptible to corrosion attack and to early failures.
Positive grid corrosion thus is a primary mode of failure of VRLA lead-acid cells. When positive grid corrosion occurs, this lowers the electrical conductivity of the cell itself. Cell failure occurs when the corrosion-induced decrease in the conductivity of the grid causes the discharge voltage to drop below a value acceptable for a particular application.
A second failure mechanism, also associated with grid corrosion, involves failure due to “grid growth.” During the service life of a lead-acid cell, the positive grid corrodes; and the corrosion products form on the surface of the grid. In most cases, the corrosion products form at the grain boundaries and grid surface of the lead-acid where the corrosion process has penetrated the interior of the “wires” of the grid. These corrosion products are generally much harder than the lead alloy forming the grid and are less dense. Due to the stresses created by these conditions, the grid alloy moves or grows to accommodate the bulky corrosion products. This physical displacement of the grid causes an increase in the length and/or width of the grid. The increase in size of the grid may be nonuniform. A corrosion-induced change in the dimension of the grid may also sometimes be called “creep”.
When grid growth occurs, the movement and expansion of the grid begins to break the electrical contact between the positive active material and the grid itself. This movement and expansion prevents the passage of electricity from some reaction sites to the grid and thereby lowers the electrical discharge capacity of the cell. As this grid growth continues, more of the positive active material becomes electrically isolated from the grid and the discharge capacity of the cell decays below that required for the particular application. The mechanical properties of the alloy thus are important to avoid undue creep during service life.
Still further, and importantly, the use of the alloys must not result in thermal runaway. VRLA cells must avoid conditions in service in which the temperature within the cell increases uncontrollably and irreversibly. It has been hypothesized that excessive water loss resulting in cell dry-out is the driving mechanism for thermal runaway in VRLA cells. This water loss can be caused by hydrogen gassing at the negative electrode or oxygen gassing at the positive electrode through the electrolysis of water, or both.
As the water content and thus the cell saturation is reduced, the oxygen recombination efficiency is increased. Since this recombination reaction is highly exothermic, this tends to heat the cell. As the temperature rises, the cell tends to generate gas; and the recombination processes become even more efficient, thereby further increasing the cell temperature. In similar fashion, water loss increases the cell electrical resistance; and such increased cell resistance increases the cell temperature, thereby further increasing water loss. The cell is in thermal runaway.
Accordingly, to avoid alloys that will push cells into thermal runaway, the effect of the alloy and its constituents on gassing at both electrodes must be taken into consideration. As is well known, antimonial alloys have been considered necessary for positive grids where the cells are required in service to endure deep discharge-charge cycling regimes. Yet, in general, although not exclusively, antimonial alloys cause thermal runaway in VRLA cells due to excessive gassing at both electrodes. Antimony thus leaches out from the positive grid as corrosion takes place, dissolving into the electrolyte, ultimately migrating to and “electroplating” onto the negative electrode. These antimony sites on the negative electrode thus become preferential to hydrogen gassing. Additionally, the presence of antimony on the negative electrode increases the self-discharge and thereby heats the cell since the self-discharge current is also reflected in the float current.
Poisoning of the positive electrode, of course, also must be avoided. Undue gassing at the positive electrode can thus lead to thermal runaway.
Further, the alloys must maintain adequate contact for electrical conductance throughout the desired service life. Otherwise, the cell will experience what has been termed as “premature capacity loss” (“PCL”). PCL can also occur through loss of contact due to cracking of the corrosion layer or from a nonconductive film generated in the corrosion layer. Because of the complexity and the substantial potential adverse effects, this is a difficult criteria to achieve in combination with the other necessary criteria.
It would also be desirable to provide positive grid alloys capable of enduring deep discharge-charge cycling regimes. Satisfying these criteria would also allow use of such alloys for both motive power and stationary VRLA applications.
Lead sulfate (PbSO4) crystals on the plates are formed as batteries discharge. These crystals become relatively difficult to charge if the plates are left in the discharged state or at open circuit for a significant period of time. Moreover, the fluid in a battery tends to evaporate over time to such an extent that upper edges of battery plates become exposed that they become susceptible to corrosion. This corrosion of the plates, especially positive plates, further deteriorates the ability of a battery to be recharged and hold a charge.
In some prior batteries and, in particular, industrial batteries, MFX (PbSbCd) is the main alloy for the positive grids. MFX is robust and has good mechanical properties and excellent corrosion resistance. However, this alloy contains Cd, which causes environmental and recycling issues. Therefore, the use of Cd-containing alloys is restricted globally because of environmental concerns.
Alloy A (PbSnCaAg) is a replacement for MFX and is used extensively in current production. It performs well in the BCI cycle life test, but overall it does not fully match the performance of MFX. The addition of Ag increases the general corrosion resistance but also increases cost and creates adhesion issues between the grids and PAM (Positive Active Materials). In particular, it shows PCL at the high rate discharge test.
What is needed in the art is a new alloy for a battery grid that adequately satisfies the diverse requirements needed for making battery grids for positive plates and, in particular, is cheaper and performs better than current Alloy A.